Soil improvement method

Abstract

To provide a soil improvement method utilizing urea and urease, allowing reduction of ammonia generated in the course of soil improvement.SOLUTION: The present invention provides a soil improvement method in which calcium carbonate is generated from carbon dioxide produced through urea and urease to solidify soil, the method characterized by further adding a phosphorus compound and a magnesium compound to produce magnesium ammonium phosphate from at least a part of the ammonia generated through urea and urease.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a soil improvement method using urea and urease, which can reduce ammonia generated in the soil improvement process.

Background Art

[0002] Conventionally, as soil strength improving materials, materials such as cement, lime-based modifiers, and polymer-based modifiers have been known, and appropriate strength modifiers have been selected according to various applications, usage conditions, etc. In recent years, in order to reduce the impact on living organisms and the environment, a soil improvement method for consolidating the ground using microorganisms as disclosed in Patent Document 1 has been used. Patent Document 1 discloses a soil improvement method in which microorganisms are introduced into the ground containing calcium, and the ground is consolidated by calcium carbonate generated by the reaction of carbon dioxide gas generated by the metabolic action of the microorganisms and calcium. Patent Document 1 uses saccharides such as glucose as a source of carbon dioxide.

[0003] In addition, a ground improvement method for improving acid resistance described in Patent Document 2 is known. Patent Document 2 discloses a cement method characterized by using urease-producing microorganisms (urea-assimilating bacteria), urea, and calcium ions to produce a metal carbonate by carbon dioxide generated from urea by the metabolism of the microorganisms.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the ground improvement method using urease, ammonia is generated together with carbon dioxide during the metabolism of urea. The generated ammonia is released into the air, deteriorating the environment due to odors and the like. In addition, there was a risk that ammonia would be metabolized into nitrate and nitrite, for which environmental standards are set, by the action of nitrifying bacteria present in the soil. Further, there was a risk that the urea decomposition reaction by urease would be reduced due to the accumulation of ammonia as a by-product.

Means for Solving the Problems

[0006] As a result of intensive studies to achieve the above problems, the present inventors have found that by adding a phosphate compound and a magnesium compound, ammonia generated by urea and urease can be reduced.

[0007] Each aspect for solving the above problems will be described. The soil improvement method of Aspect 1 is a soil improvement method in which calcium carbonate is generated from carbon dioxide generated by urea and urease to consolidate the soil, and further, by adding a phosphate compound and a magnesium compound, magnesium ammonium phosphate is generated from at least a part of the ammonia generated by urea and urease. It is characterized by that.

[0008] Aspect 2 is the soil improvement method according to Aspect 1, wherein the pH of the soil is 6 or more and 10 or less. Aspect 3 is the soil improvement method according to Aspect 1 or 2, wherein the phosphate compound is an alkali metal salt of phosphoric acid.

Effects of the Invention

[0009] According to the present invention, in the soil improvement method using urea and urease, ammonia generated in the soil improvement process can be reduced.

Modes for Carrying Out the Invention

[0010] Hereinafter, embodiments in which the soil improvement method of the present invention is embodied will be described. The soil improvement method of this embodiment is applied to the calcium carbonate method, which is a soil improvement method for solidifying soil by generating calcium carbonate from carbon dioxide generated by urea and urease. In the present invention, in such a soil improvement method, by further adding a phosphate compound and a magnesium compound, a salt form of magnesium ammonium phosphate is generated from ammonia, which is an odor component generated by urea and urease.

[0011] (Calcium carbonate method) The calcium carbonate method is a method for solidifying the ground by calcium carbonate precipitated by the reaction between carbon dioxide generated by microbial metabolism or enzymatic reaction and a calcium source in the soil pores or added calcium source. For the calcium carbonate method, known methods and conditions are appropriately adopted. Urea is used as the carbon dioxide generation source. Urease hydrolyzes urea to generate carbon dioxide and ammonia.

[0012] As the urease for decomposing urea, the urease enzyme itself may be applied (EICP method), or urease-producing microorganisms (urea-assimilating bacteria) (MICP method) may be applied. As the urease-producing microorganisms, naturally occurring microorganisms present in the soil to be applied may be used, or separately cultured urease-producing microorganisms may be applied to the soil. Among these, when prioritizing cost and the like, the method using microorganisms is preferably applied. Also, when prioritizing the reaction rate, the method using enzymes is preferably applied.

[0013] As the calcium source, those existing in the soil pores may be applied, or a calcium source may be added separately. Examples of the calcium source to be added include oxides, chlorides, hydroxides of calcium, salts with inorganic acids or organic acids, etc. Specific examples of the calcium source include, for example, lime, calcium chloride, calcium nitrate, calcium acetate, etc.

[0014] The purpose of soil improvement using the calcium method is not particularly limited. For example, improvement of handleability, improvement of surface strength for the purpose of preventing dust and runoff soil, improvement of ground strength, liquefaction countermeasures, etc. can be mentioned.

[0015] (Phosphate compound) The phosphate compound is not particularly limited as long as it can form magnesium ammonium phosphate from the ammonia generated by the above-mentioned urease and the magnesium compound described below. From the viewpoints of storage stability, handleability, etc., it is preferably an inorganic phosphate compound. Examples of the inorganic phosphate compound include alkali metal salts of phosphoric acid, alkaline earth metal salts of phosphoric acid, etc. Specific examples of the alkali metal constituting the alkali metal salt include, for example, sodium, potassium, lithium, etc. Examples of the alkaline earth metal constituting the alkaline earth metal salt include metals corresponding to Group 2 elements, such as calcium, magnesium, beryllium, strontium, barium, etc. These can be appropriately selected in consideration of conditions such as the pH of the soil to which they are applied. As will be described later, when the soil pH after construction is acidic, it is preferable to apply an acidic phosphate. On the other hand, when the soil pH after construction is alkaline, it is preferable to apply an alkaline phosphate. Since 2 moles of ammonia are generated from 1 mole of urea by urease, the appropriate application amount of the phosphate compound is preferably about 2 moles of phosphoric acid per 1 mole of urea blended.

[0016] (Magnesium compound) The magnesium compound, when blended with the phosphate compound, generates magnesium ammonium phosphate from ammonia produced by urea and urease. The magnesium compound is not particularly limited as long as it can generate magnesium ammonium phosphate from the ammonia produced by the above-mentioned urease and the above-mentioned phosphate compound. Examples of the magnesium compound to be added include oxides, chlorides, hydroxides, salts with inorganic acids or organic acids, etc. of magnesium. From the viewpoints of storage stability, handleability, etc., it is preferably an inorganic magnesium compound. Specific examples of the magnesium compound include magnesium sulfate, magnesium chloride, magnesium hydroxide, magnesium carbonate, magnesium oxide, etc. Since 2 moles of ammonia are generated from 1 mole of urea by urease, the appropriate application amount of the magnesium compound is preferably about 2 moles of magnesium per 1 mole of urea to be blended.

[0017] (pH) In the soil improvement method of the present embodiment, the pH of the soil to be applied is weakly acidic to weakly alkaline. When improving the strength of the improved soil, weakly acidic conditions are applied. Also, when reducing the outflow of ammonium salts due to rainfall, groundwater, etc. and when reducing the decrease in soil strength after flooding, neutral to weakly alkaline conditions are applied. Note that weakly acidic indicates a range of pH 4 or more and less than 6, neutral indicates a range of pH 6 or more and less than 8, and weakly alkaline indicates a range of pH 8 or more and 10 or less. Among these, from the viewpoint of preventing the outflow of ammonium salts due to rainfall, groundwater, etc. and reducing the decrease in soil strength after flooding, the pH is preferably 6 or more and 10 or less, and more preferably 7 or more and 9.5 or less.

[0018] The pH of the soil may be adjusted by applying the above-mentioned phosphates, etc. in addition to known buffers. For example, when applying a compound whose aqueous solution shows acidity, such as potassium dihydrogen phosphate or sodium dihydrogen phosphate, as the above-mentioned phosphate compound, acidic conditions can be achieved. When applying a compound whose aqueous solution shows alkalinity, such as dipotassium hydrogen phosphate or disodium hydrogen phosphate, as the above-mentioned phosphate compound, alkaline conditions can be achieved.

[0019] (Function of this Embodiment) The soil improvement method of this embodiment is applied to the calcium carbonate method, which is a soil improvement method for solidifying soil by generating calcium carbonate from carbon dioxide generated by urea and urease. In such a soil improvement method, by further adding the above-mentioned phosphate compound and magnesium compound, magnesium ammonium phosphate is generated from at least a part of ammonia, which is an odor component generated by urea and urease. Thereby, ammonia, which is an odor component generated by urease, can be reduced.

[0020] (Effect of this Embodiment) The effect of this embodiment will be described. (1) This embodiment adopts a configuration in which a phosphate compound and a magnesium compound are further added in a soil improvement method for solidifying soil by generating calcium carbonate from carbon dioxide generated by urea and urease. Thereby, magnesium ammonium phosphate in salt form is generated from at least a part of ammonia generated by urea and urease.

[0021] Therefore, ammonia, which is an odor component generated during soil improvement, can be reduced. In addition, odor reduction and the like can be achieved, and deterioration of the surrounding environment can be suppressed. In addition, by suppressing the accumulation of ammonia, which is a by-product of the decomposition reaction of uric acid, a decrease in the urea decomposition reaction by urease can be suppressed. In addition, ammonia may be metabolized into nitrate and nitrite whose environmental standards are defined by the action of nitrifying bacteria present in the soil. By the method of this embodiment, the generation of nitrate and nitrite can be reduced.

[0022] (2) In the soil improvement method, when the pH of the soil is defined to be neutral to weakly alkaline (pH 6 or more and 10 or less), the outflow of ammonium salts due to rainfall, groundwater, etc. can be reduced. In addition to ammonium salts, the inflow of nitrates or nitrites generated by the metabolism of microorganisms from ammonium salts into groundwater can be reduced. In addition, a decrease in soil strength after flooding can be reduced.

[0023] (3) In the soil improvement method, when the pH of the soil is specified to be weakly acidic (pH 4 or more and less than 6), the strength of the improved soil can be improved. (4) When an alkali metal salt of phosphoric acid is applied as the phosphate compound, magnesium ammonium phosphate can be precipitated under cost - advantageous conditions. Further, by applying a metal dihydrogen phosphate or a metal hydrogen phosphate, it can function as a buffer for adjusting the pH of the soil.

[0024] (Modified example) Note that the above - mentioned embodiment may be modified as follows. The above - mentioned embodiment and the following modified examples can be implemented in combination with each other within a range where there is no technical contradiction.

[0025] · The soil improvement method of the above - mentioned embodiment can be applied to a configuration in which ammonia is generated by urea and urease. Within the range where the effects of the present invention can be obtained, it does not prevent the combined use of organic substances such as monosaccharides such as glucose, disaccharides such as sucrose, and polysaccharides such as starch as a carbon dioxide generation source.

Example

[0026] Hereinafter, the present invention will be described in more detail based on examples. Note that the present invention is not limited to the following examples, and various modifications can be made without departing from the idea of the present invention.

[0027] <Test example 1: Ammonia suppression test> In the soil improvement method using the calcium method, the ammonia suppression effect when a phosphate compound and a magnesium compound are added was tested.

[0028] (Amount of vaporized ammonia) Sample soils were prepared by mixing Toyoura silica sand and each material at the ratios shown in Tables 1 and 2. 500 g of the prepared sample soil was transferred to a 2-L bag and cured at room temperature for one week in a state where it was also sufficiently filled with air. The ammonia concentration in the air inside the bag for each example was measured using an ammonia detector tube (manufactured by Gastec Corporation: 5 - 100 ppm, 0.05 - 1.6%). The results are shown in Tables 1 and 2.

[0029] (Hardness Test) Sample soils were prepared by mixing Toyoura silica sand and each material at the ratios shown in Tables 1 and 2. They were transferred to containers with a predetermined dosage, filled to a thickness of 50 mm, and cured at room temperature for one week.

[0030] The strength of the cured sample soil was measured using a penetrometer in accordance with the Geotechnical Society of Japan standard: JGS1441 "Soil Hardness Test Method". Three measurements were taken for each sample soil, and the average value was obtained. The results are shown in Tables 1 and 2.

[0031] (pH of Sample Soil) For each sample soil, water at a ten-fold dosage was added, mixed well, and then the supernatant was taken out and the pH was measured. The results are shown in Tables 1 and 2.

[0032]

Table 1

[0033]

Table 2

[0034] <Reference Test 1: Test of Hardness, etc. of Sample Soil after Sprinkling Water> For the cured sample soils of the above Examples 1 to 3 and Comparative Examples 1 and 2, further watering treatment was performed. Thereafter, the strength of the sample soil, the pH of the eluate, and the concentration of water-soluble ammonia (concentration of ammonium salt) were measured.

[0035] The watering treatment was carried out for 60 minutes at a dosage of 50 mm / h using atomization. The strength of the sample soil one day after watering was measured in the same manner as in Test Example 1. The test results are shown in Table 3. Also, the pH and ammonia amount (ammonium ion concentration) of the leachate eluted from the sample soil during the above watering treatment were measured. The ammonium ion concentration in the leachate was measured using ion chromatography. The test results are shown in the "Leachate pH" column and "Leachate Ammonium Ion Concentration" column of Table 3, respectively.

[0036]

Table 3

Claims

Claim 1 A soil improvement method for consolidating soil by producing calcium carbonate from carbon dioxide generated by urea and urease, wherein a soil improvement method is provided in which a magnesium ammonium phosphate is produced from at least a part of ammonia generated by urea and urease by further adding a phosphate compound and a magnesium compound. Claim 2 The soil improvement method according to claim 1, wherein the pH of the soil is 6 or more and 10 or less. Claim 3 The soil improvement method according to claim 1, wherein the phosphate compound is an alkali metal salt of phosphoric acid.

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